1. Field of the Invention
This invention relates to nuclear reactors and, in particular, to an apparatus for locating defective fuel elements.
2. Description of the Prior Art
In water cooled heterogeneous reactors, a multiplicity of elongated fuel elements and control element guide tubes are arranged, as a closely spaced array, in a unified structure called a fuel assembly. The reactor core is generally comprised of a lattice of vertically disposed fuel assemblies.
Each of the elongated fuel elements, which are alternatively characterized as fuel rods, tubes or pins, contain nuclear fuel encapsulated by a thin cladding, plugged at its ends, that prevents erosion of the fuel and the release of fission products into the reactor coolant. Aluminum or its alloys, the stainless steels and zirconium are typical clad materials.
Plenum chambers and clearances are provided within the fuel elements to accommodate fission product gas released from the fuel, differential thermal expansion between the cladding and the fuel, and fuel density changes during burnup. The plenums are generally located at the ends of the fuel element and contain plenum springs which maintain the nuclear fuel in a fixed relationship. In some cases, the fuel elements are initially pressurized with a gas, typically helium, to minimize clad creep during prolonged periods of operation at high reactor coolant system pressures.
The fuel element cladding is designed to withstand the effects of the reactor operating environment including those due to coolant hydraulics, reactor temperature and pressure, fission gas pressure, fuel expansion, and irradiation growth. Some cladding defects, which permit the escape of radioactive fission products into the fluid coolant or moderator, however, may be expected to occur during the operating life of the reactor. Although purification systems are designed to remove the maximum amount of radioactivity expected to occur due to cladding defects, it may be desirable or necessary to detect and replace defective or "failed" fuel elements. Hence, it is important to have reliable means for locating defective fuel elements.
On one hand, locating a defective fuel element within a fuel assembly is extremely difficult since an assembly is radioactive and may contain hundreds of closely spaced fuel elements and guide tubes. On the other hand, disassembly and reassembly of irradiated fuel assemblies is time consuming and may, in itself, result in fuel element damage.
In reactors utilizing liquid coolants, a number of devices and techniques have been proposed for locating individual defective elements within the fuel assembly based upon detection and analysis of vibration, temperature differentials or ultrasonic phenomena.
These prior art detection devices and techniques have depended, in general, upon at least partial disassembly of a fuel assembly. Moreover, great dependence has been placed, in the prior art, on the dynamics of thermodynamic changes of state of the fluid which has leaked into the defective fuel element, typically boiling or condensation or both.
In order to facilitate the location of failed fuel elements within a fuel assembly, the development of a reliable method and apparatus which neither requires disassembly of a fuel assembly nor is dependent upon boiling or condensation of fluid within the fuel elements has been desired.